1. Field of the Invention
The present invention essentially relates to a signal transmission system and, more particularly, to a signal transmission system for transferring long messages, such as user packets, in a mobile communication system whose transmission paths have relatively low reliability.
2. Description of the Related Art
Hitherto, various re-transmission control systems, such as a Go-Back-N (GBN) system and a Selective-Repeat (SR) system, have been proposed and actually employed for the data-link layer protocol such as HDLC (High-Level Data Link Control) or the like.
In the GBN system, a plurality of frames are transmitted simultaneously, and all frames are transmitted again even if any of the frames has failed to reach a receiving station. In the SR system, a plurality of frames are transmitted simultaneously, and only one or more of the frames, which have failed to reach a receiving station, are transmitted again to the station.
In a mobile transmission system having transmission paths in which fading may occur and which are therefore unreliable, the failure rate of transferred words (or control signals), which amounts to only about 10 bytes, may be reduced to about 10.sup.-2 by using, for example, error correcting codes. When data of about 2 kilobytes, is transferred, however, the failure rate of data will be 0.86, if not affected by the failure rate determined for each subsection of the mobile communication system. Obviously, the mobile communication system has but extremely low reliability.
In other words, the mobile communication system is sufficiently reliable as long as re-transmission control is performed on a relatively small amount of data, such as control signals. However, when the re-transmission control is performed on a large amount of data, such as a user packet, the data signals must be transferred many times, and many re-transmission request signals must be transmitted from the receiving station. Consequently, the transmission paths of the system become so "overcrowded" that data can scarcely be transmitted through them.
In order to avoid the overcrowding of the paths, a great amount of data may be divided into small blocks, and these data blocks may be transferred through the paths under the conventional re-transmission control. If this method is used, however, a transmission sequence order number must be added to each data block. Further, the ID number assigned to the mobile station to which the data block is to be transferred must also be added to the data block, particularly when the mobile system includes many mobile stations. When an order number and an ID number are added to each data block, the operating efficiency of the mobile system decreases inevitably.
Furthermore, if the conventional re-transmission control is applied in the mobile communication system, it will be necessary to divide a message into blocks and reconstruct it in accordance with the instruction of upper layers. Obviously, complex data-processing must be performed in order to divide and reconstruct a message.